A) Field of the Invention
This invention relates generally to a solid state image pickup device and a driving method therefor, and particularly to a solid state image pickup device with a novel pixel structure and a solid state image pickup device drive method with a novel pixel signal reading operation.
B) Description of the Related Art
MOS type, CCD type and CMOS type solid state image pickup devices are known as conventional solid state image pickup devices.
FIG. 6 shows the structure of a conventional MOS type solid state image pickup device.
A number of pixels PX are disposed in a matrix layout on the surface of a semiconductor substrate. Each pixel PX includes one photodiode PD as a photosensitive element and one MOS field effect transistor MOSFET for reading charges accumulated in the photodiode PD. In the arrangement illustrated in FIG. 6, the cathode of the photodiode PD constitutes a charge accumulation region and is connected to the source region of MOSFET. A row select signal line 103 is connected to the gate of MOSFET and the drain of MOSFET is connected to a read signal line 105.
The row select signal lines 103 are connected to a vertical shift register VSR and sequentially receive row select signals. The read signal lines 105 are connected via column select transistors 107 to an output amplifier AMP. The control electrodes of the column select transistors 107 are connected to a horizontal shift register HSR and sequentially receive column select signals. A timing generator 109 supplies a timing signal to the horizontal shift register HSR and vertical shift register VSR. These registers HSR and VSR start their operations upon reception of a V-Start signal and an H-Start signal, respectively.
While a single pixel row is selected by the vertical shift register VSR, the horizontal shift register HSR sequentially selects the respective columns and supplies the charges of one row to the output amplifier AMP.
This arrangement is similar to the arrangement of a DRAM whose memory cell is constituted of one MOSFET and one capacitor. Although this arrangement uses MOSFET, it does not have an amplification function and is thus called also a passive sensor. The structure shown in FIG. 6 has been manufactured normally using n-MOS processes.
If the electrical characteristics of switching MOSFET's have fluctuations, outputs of pixels will have variations even with the same incident light amount so that fixed pattern noises are generated.
An image cannot be picked up with all pixels at the same time so that the image of a moving subject drifts. Charges accumulated in all pixels are hard to be electronically cleared at the same time.
FIG. 7 shows the structure of an interline CCD (IT-CCD) which is used most often among solid state image pickup devices.
The arrangement of IT-CCD having a matrix layout of pixels PX each constituted of a photodiode PD and a MOSFET is similar to that shown in FIG. 6. In place of the read signal line, a vertical charge coupled device VCCD is disposed along a pixel column. One end of VCCD is connected to a horizontal charge coupled device HCCD. The output end of HCCD is connected to a floating diffusion amplifier FDA.
In IT-CCD, signal charges accumulated in the cathode regions of photodiodes PD are transferred through MOSFET's to VCCD's, HCCD, and to FDA along the charge transfer paths only in semiconductor. Each VCCD has a number of transfer stages and can hold charges. Charges can thus be read into VCCD's from a number of pixels simultaneously. VCCD is for example driven by four-phase drive signals φV1 to φV4. HCCD is for example driven by two-phase drive signals φH1 and φH2 at high speed. Still images without drift can therefore be output.
A light shielding film is disposed above the charge transfer paths of VCCD's and HCCD to prevent light from entering into the charge transfer paths. A highly sensitive solid state image pickup device can be realized which is less affected by noises. Improvement on the image quality is realized by incorporating a complete depletion type photodiode structure. Since charges accumulated in pixels can be moved to VCCD's at the same time via transfer gates, a so-called completely electronic shutter can be realized.
IT-CCD requires a high drive voltage and a high consumption power so that it is difficult to be driven with a single power source. IT-CCD is manufactured by dedicated specific processes different from generalized CMOS processes. Since charges read from photodiodes PD's are output via VCCD's and HCCD, random access of pixels is difficult.
FIG. 8 shows a CMOS type solid state image pickup device. Although the arrangement for just one pixel is shown in FIG. 8, a number of pixels PX are disposed in a matrix layout similar to those pixels shown in FIGS. 6 and 7.
Each pixel PX is constituted of: a photodiode PD; a source follower amplifier SFA for reading and amplifying charges accumulated in the photodiode PD; and a reset transistor RT. The source follower amplifier SFA includes an amplifier transistor 121 whose gate receives a signal voltage and a transfer transistor 123.
The respective current terminals of the transfer transistor 123 and reset transistor RT are connected to a power source line 117. One terminal of the amplifier transistor 121 is connected to a read signal line 113. The gate electrode of the transfer transistor 123 is connected via a row select signal line 111 to a vertical shift register VSR. The gate electrode of the reset transistor RT is connected to a reset signal line 115.
The read signal line 113 is connected via a noise canceler 131 to one terminal of a column select transistor 133. The other terminal of the column select transistor 133 is connected to an output amplifier AMP to output an image signal. The gate electrode of the column select transistor 133 is connected to a horizontal shift register HSR.
With the widespread of portable information terminals, cameras for inputting images to personal computers (PC's), and compact digital still (DS) cameras, attention has been paid to compact solid state image pickup devices of low power consumption. CMOS solid state image pickup devices based on CMOS processes are developed which can be driven by a single power supply and at low power consumption as different from CCD solid state image pickup devices. With the CMOS arrangement, on-chip peripheral circuits can be realized easily and low power consumption can be realized.
These merits promote the improvement on the characteristics of and the practical application of CMOS type solid state image pickup devices. Low noises are realized by providing an amplifier for each pixel of a CMOS solid state image pickup device. This device is called also an active sensor because each pixel has an active (amplifying) element. This device however requires in addition to one photodiode, three or more transistors (MOSFET's) per pixel.
As the number of MOSFET's per pixel increases, the operation margin of the photodiode becomes severe, resulting in a difficulty of realizing high sensitivity and high density of pixels. In a photodiode type CMOS solid state image pickup device, an ohmic contact exists between the read circuit and pixel so that it is difficult to lower the impuirty concentration of the entire charge accumulation region and thus difficult to realize a complete depletion type photodiode. From this reason, fixed pattern noises (FPN) and reset noises are generated when photodiodes are reset to a fixed potential. Fixed pattern noises are generated because of variations in the depletion layer capacitance of the photodiodes. Reset noises are generated by thermal fluctuation of the channel resistances of the reset transistors RT. Since the device is driven by XY sequential addressing, the image of a moving subject drifts. An electronic shutter function or mechanical shutter-less camera is difficult to be realized.
The present inventor proposed a non-volatile solid state image pickup device having a non-volatile memory function in each pixel in a U.S. patent application Ser. No. 10/100,069 based on Japanese Patent Application No. 2001-083374, which is incorporated herein by reference.
As described above, conventional solid state image pickup devices have such merits as well as demerits.